How do we know the doctrine of being in Christ is true?

The doctrine of being in Christ is foundational to Reformed theology. As stated in John 15, without abiding in Christ, believers can do nothing. This truth is confirmed throughout Scripture; for example, 2 Corinthians 5:17 declares that anyone who is in Christ is a new creation. The work of the Holy Spirit is also vital here, as He indwells believers and enables them to live out their new identity. The belief is not a matter of human will or effort but is entirely a divine work motivated by God’s grace, emphasizing God's sovereignty in salvation.